The precise mechanism whereby osteoblasts mediate osteoclastic bone resorption is unclear. One widely-held hypothesis is that activated osteoblasts secrete cytokines which directly or indirectly influence osteoclast formation or function. Although the nature of these putative cytokines is unknown, compelling in vivo and in vitro data have emerged to support a role for colony-stimulating factor-1(CSF-1- as an osteoblast-derived factor involved in modulating osteoclast formation and function. Thus, in vivo deficiency of CSF-1 in the op/op osteopetrotic mouse causes a failure of osteoclast formation and bone resorption while in vitro studies have demonstrated that CSF-1 is critical for the proliferation and differentiation of osteoclast progenitors, that CSF-1 stimulates bone resorption in the fetal mouse metacarpal assay, and that CSF-1 receptors are present on osteoclasts. CSF-1 is synthesized as both soluble and cell surface proteins, and osteoblasts are known to synthesize both isoforms constitutively and in response to osteotropic agents. However, little is known about their physiologic significance in bone remodeling. Although the downstream signaling events that occur following CSF-1 receptor c-fms activation have been described in other cells, little information is available in osteoclasts. The goals of this proposal is therefore to study the effects of soluble and cell-surface forms of CSF-1 on cell-signaling by characterizing those proteins uniquely tyrosine phosphorylated in response to the treatment of growth factors and by examining their dose- and time-dependent effects on phosphatidylinositol-3 kinase. (PI 3-K) kinase activity in osteoclast precursors and mature osteoclasts. These studies will help to understand osteoclast signaling events following CSF-1 stimulation, and distinguish two forms of CSF-1 in their different roles on bone remodeling.